Photophysical properties of coumarin-7 dye: role of twisted intramolecular charge transfer state in high polarity protic solvents

Photophysical studies on coumarin-7 (C7) dye in different protic solvents reveal interesting changes in the properties of the dye on increasing the solvent polarity (Deltaf; Lippert-Mataga solvent polarity parameter) beyond a critical value. Up to Deltaf approximately 0.31, the photophysical properties of the dye follow good linear correlations with Deltaf. For Deltaf > approximately 0.31, however, the photophysical properties, especially the fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)) and nonradiative rate constants (k(nr)), undergo large deviations from the above linearity, suggesting an unusual enhancement in the nonradiative decay rate for the excited dye in these high polarity protic solvents. The effect of temperature on the tau(f) values of the dye has also been investigated to reveal the mechanistic details of the deexcitation mechanism for the excited dye. Studies have also been carried out in deuterated solvents to understand the role of solute-solvent hydrogen bonding interactions on the photophysical properties of the dye. Observed results suggest that the fluorescence of the dye originates from the planar intramolecular charge transfer (ICT) state in all the solvents studied and the deviations in the properties in high polarity solvents (Deltaf > approximately 0.31) arise due to the participation of a new deexcitation channel associated with the formation of a nonfluorescent twisted intramolecular charge transfer (TICT) state of the dye. Comparing present results with those of a homologous dye coumarin 30 (C30; Photochem. Photobiol., 2004, 80, 104), it is indicated that unlike in C30, the TICT state of the C7 dye does not experience any extra stability in protic solvents compared to that in aprotic solvents. This has been attributed to the presence of intramolecular hydrogen bonding between the NH group (in the 3-benzimidazole substituent) of the C7 dye and its carbonyl group, which renders an extra stability to the planar ICT state, making the TICT state formation relatively difficult. Qualitative potential energy diagrams have been proposed to rationalize the differences observed in the results with C7 and C30 dyes in high polarity protic solvents.     

The effect of solvent polarity on the photophysical properties of 4-cyano-(4'-methylthio)diphenylacetylene: a prototypic donor-acceptor system

The photophysical properties of the target compound are extremely sensitive to changes in solvent polarity since the lowest-energy excited states possess considerable charge-transfer character. Excitation results in a greatly increased dipole moment, with the resultant excited singlet state retaining a lifetime of ca. 1 ns in all solvents. Radiative decay involves coupling between the lowest-energy excited singlet state and both the ground state and an upper excited singlet state. The level of coupling to the upper singlet decreases in non-polar solvents, presumably due to symmetry factors. The radiative rate constant decreases smoothly with increasing solvent polarity function as the molecule acquires an ever increasing dipolar character. Non-radiative decay includes both intersystem crossing and internal conversion, but the former process dominates in polar solvents. The excited singlet state lifetime is very weakly dependent upon temperature in the solid state. However, in polar solutions where the Stokes' shift decreases with decreasing temperature, there is clear evidence for an activated process. This is believed to involve coupling to the upper-lying singlet excited state.     

Effect of solvent on the excited-state photophysical properties of curcumin

Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (delta mu = 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S1) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50-350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500-1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl3 and dimethyl sulfoxide-D6 also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S1) and triplet (T1) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (phi T < or = 0.12). Both the fluorescence and triplet quantum yields being low (phi f + phi T < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.     

Intriguing H-aggregate and H-dimer formation of coumarin-481 dye in aqueous solution as evidenced from photophysical studies

Photophysical properties of coumarin-481 (C481) dye in aqueous solution show intriguing presence of multiple emitting species. Concentration and wavelength dependent fluorescence decays and time-resolved emission spectra and area-normalized emission spectra suggest the coexistence of dye monomers, dimers, and higher aggregates (mostly trimers) in the solution. Because of the efficient intramolecular charge transfer (ICT) state to twisted intramolecular charge transfer (TICT) state conversion, the dye monomers show very short fluorescence lifetime of ~0.2 ns. Fluorescence lifetimes of dimers (~4.1 ns) and higher aggregates (~1.4 ns) are relatively longer due to steric constrain toward ICT to TICT conversion. Observed results indicate that the emission spectra of the aggregates are substantially blue-shifted compared to monomers, suggesting H-aggregation of the dye in the solution. Temperature-dependent fluorescence decays in water and time-resolved fluorescence results in water-acetonitrile solvent mixtures are also in support of the dye aggregation in the solution. Though dynamic light scattering studies could not recognize the dye aggregates in the solution due to their small size and low concentration, fluorescence up-conversion measurements show a relatively higher decay tail in water than in water-acetonitrile solvent mixture, in agreement with higher dye aggregation in aqueous solution. Time-resolved fluorescence results with structurally related non-TICT dyes, especially those of coumarin-153 dye, are also in accordance with the aggregation behavior of these dyes in aqueous solution. To the best of our knowledge, this is the first report on the aggregation of coumarin dyes in aqueous solution. Present results are important because coumarin dyes are widely used as fluorescent probes in various microheterogeneous systems where water is always a solvent component, and the dye aggregation in these systems, if overlooked, can easily lead to a misinterpretation of the observed results.     

The effect of solvent polarity on the molecular surface properties and adhesion of Escherichia coli

The elasticity and molecular surface characteristics of Escherichia coli JM109 were investigated via atomic force microscopy (AFM) in solvents expressing different polarities. The nature of bacterial adhesion and surface characteristics was probed in formamide, water, and methanol, with dielectric constants of 111, 80, and 33, respectively. Solvent polarity affected the elasticity of the bacterium, the conformation of the cell surface biopolymers, the height of the surface biopolymers, and measured adhesion forces between the bacterium and silicon nitride. By applying the Hertz model to force-indentation data, we determined that the Young's modulus was greatest in the least polar solvent, with values of 182 +/- 34.6, 12.8 +/- 0.1, and 0.8 +/- 0.3 MPa in methanol, water, and formamide, respectively. The thickness of the biopolymer brush layer on the bacterial surface was quantified using a steric model, and these values increased as polarity increased, with values of 27, 93, and 257 nm in methanol, water, and formamide, respectively. The latter results suggest that highly polar conditions favor extension of the biopolymer brush layer. Cross-sectional analysis performed on tapping mode images of the bacterial cells in methanol, water, and formamide further supported this hypothesis. The image height values are larger, since the image analysis measures the height of the bacterium and the polymer layer, but the trend with respect to solvent polarity was the same as was obtained from the steric model of the brush length. Measured adhesion forces scaled inversely with solvent polarity, with greatest adhesion observed in the least polar solvent, methanol. The combined conformational changes to the bacterial surface and biopolymer layer result in different presentations of macromolecules to a substrate surface, and therefore affect the adhesion forces between the bacterial molecules and the substrate. These results suggest that polarity of the solvent environment can be manipulated as a design parameter to control or modify the bacterial adhesion process.     

Orientational effect on the photophysical properties of quaterthiophene-C60 dyads

Two quaterthiophene-[60]fullerene dyads in which C60 is singly (4TsC) or doubly (4TdC) connected to the inner beta-position of the terminal thiophene rings have been synthesized. The electronic properties of these donor-acceptor compounds were analyzed by UV/Vis spectroscopy and cyclic voltammetry, and their photophysical properties in solution and in the solid state by (time-resolved) photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy. Both the flexible and geometrically constrained 4TsC and 4TdC dyads exhibit photoinduced charge transfer from the quaterthiophene to the fullerene in toluene and o-dichlorobenzene (ODCB). In toluene, charge transfer occurs in both dyads by an indirect mechanism, the first step of which is a singlet-energy transfer from the 4T(S1) state to the C60(S1) state. In the more polar ODCB, direct electron transfer from 4T(S1) competes with energy transfer, and both direct and indirect charge transfers are observed. The geometrical fixation of the donor and acceptor chromophores in 4TdC results in rate constants for energy and electron transfer that are more than an order of magnitude larger than those of the flexible 4TsC system. For both dyads, charge recombination is extremely fast, as inferred from picosecond-resolved temporal evolution of the excited state absorption of the 4T.+ radical cation both in toluene and ODCB.     

The relation between the nonlinear dielectric effect and solvent polarity

The relation between solvent polarity expressed through the Dimroth-Reichardt spectroscopic parameter E _T (30) and the nonlinear dielectric effect (NDE) expressed through the parameter /E² is demonstrated where is a change in the electric permittivity of a solvent in an external strong electric field E. Both E _T (20) and /E², determined in quite different ways, are extremely sensitive to the dielectric properties of a solvent which depend on molecular interactions. Linear correlations between /E² and E _T (30) have been found for n-alkanols representing hydrogenbond donor solvents, and for halogenobenzenes which are dipolar, aprotic, weakly-associated solvents.Part of this work was presented at The 22^nd International Conference on Solution Chemistry in Linz, Austria, July 1991.     

Solvent effect on the photophysical properties of the anticancer agent ellipticine

This paper investigates how solution conditions, especially solvent polarity and hydrogen bonding, affect the fluorescence of ellipticine, a natural plant alkaloid with anticancer activity. A total of 16 solvents that cover a wide range of polarities were tested. The ultraviolet (UV) absorption and fluorescence emission of ellipticine were found to be solvent dependent. The absorption and emission maximum shifted to higher wavelengths (red shift) with increased solvent polarity. The difference in absorption and emission maximum (Stokes' shift) was large, approximately 10,000-11,000 cm-1, in polar solvents (with orientation polarizability Deltaf>0.2) but unusually small, approximately 8900 cm-1, in nonpolar solvents (hexane and cyclohexane). Large Stokes' shifts were due to an intramolecular charge transfer (ICT), which was enabled by large solvent polarity and hydrogen bonding of ellipticine with the solvents. Two transitions were found in the Lippert-Mataga plot between (1) nonpolar and semipolar solvents and between (2) semipolar and polar solvents. The first transition reflected the formation of hydrogen bonds between ellipticine and the solvents whereas the second transition indicated that ellipticine underwent an ICT. In addition, the larger extinction coefficients and the longer lifetime of ellipticine obtained in protic solvents were attributed to the formation of stronger hydrogen bonds. The photophysical response of ellipticine to changes in solvent polarity and hydrogen bond formation could be used to infer the location of ellipticine in a heterogeneous medium, namely liposomes in aqueous solution. A relatively large red shift of emission in liposomes indicated that ellipticine may be in a more polar environment with respect to the lipid bilayer, possibly close to the hydrophilic interface.     

Substitution and solvent effects on the photophysical properties of several series of 10-alkylated phenothiazine derivatives

The photophysical properties of several 2-substituted, 10-alkylated phenothiazines were measured in several solvents to investigate the relevance of the molecular structure in their photophysics and consequent photochemistry. Because the interaction modes of each drug and its corresponding species strongly depend on the variety of microenvironments in the cells, the properties of each one of these species must also be determined separately to understand fully the mechanism of action of the drug and the mechanism of its side effects. Information on the chemical interactions of the different species at the cellular level can be inferred from the corresponding electronic properties. In this work, we present absorption, steady-state, and time-resolved emission, laser flash photolysis, and quantum theoretical results for the ground state, the first excited singlet and triplet states, and the cation radical of promazine hydrochloride (PZ), 2-chlorpromazine hydrochloride (CPZ), 2-trifluoromethylpromazine hydrochloride (TFMPZ), 2-trifluoromethylperazine dihydrochloride (TFMP), 2-thiomethylpromazine (TMPZ), and thioridazine hydrochloride (TR). The corresponding nonalkylated phenothiazines are included as references. The photophysical properties of this drug family depend more on the solvent and the 2-substituents than on the dialkylaminopropyl chain. The largest effect was found for the triplet state of the 2-halogenated derivatives in phosphate buffer (PBS). Both the quantum yield and the lifetime of this intermediate drop to less than 5% of the corresponding value in organic solvents. The triplet state of halogenated promazines is efficiently quenched by a proton-transfer mechanism, and the rate of this quenching correlates very well with the phototoxicity of the promazine drugs. Therefore, we postulate that this species is directly related to the phototoxic side effect of neuroleptic drugs.     

Role of hydrogen bonding in the photophysical properties of isomeric tetrapyridylporphyrins in aprotic solvent

Extensive photophysical properties of isomeric tetra-2-pyridylporphyrin (TpyP(2)), tetra-3-pyridylporphyrin (TpyP(3)), and tetra-4-pyridylporphyrin (TpyP(4)) have been studied in the presence of a series of phenols of increasing hydrogen bonding power in dichloromethane solution by employing UV/vis spectroscopy; steady-state, time-resolved fluorescence spectroscopy; and transient absorption spectroscopic techniques. The change of absorption spectra of all three porphyrins as a function of different phenol concentrations established the preference of hydrogen bonded complex formation to the peripheral pyridyl nitrogen rather than the pyrrole nitrogen of the porphyrin macrocycle. The fluorescence behaviors of the porphyrins which were observed upon addition of different phenols point to a marked dependence on the nature of the added phenols. Phenols with an electron withdrawing group do not quench the fluorescence of porphyrins, whereas phenols with an electron donating group quench the singlet porphyrin both in static and dynamic pathways. A remarkable difference in quenching behaviors of singlet excited porphyrin by 4-methylphenol (4-MePhOH) and 4-MeOPhOH/4-EtOPhOH (4-EtOPhOH = 4-ethoxyphenol) are observed. The quenching of singlet excited porphyrins by 4-MePhOH is attributed to be purely static in nature, and the H-bond provides a strong nonradiative channel to singlet excited porphyrins. However, the quenching of singlet excited porphyrins by 4-MeOPhOH/4-EtOPhOH is mostly dynamic, and it is ascribed to be the reductive quenching of single excited porphyrins. Picosecond transient absorption study with TpyP(2) and 4-MeOPhOH provides the evidence of porphyrin radical anion and phenol radical cation of equal lifetime, which indicates the fact that electron transfer occurs from phenol to singlet excited porphyrin. The temperature effect on dynamic quenching by 4-MeOPhOH/4-EtOPhOH and kinetic deuterium isotope effect established the reaction to be a photoinduced concerted proton coupled electron transfer.     

Quantum-chemical investigation of the effect of solvent polarity on the direction of sulfonation of pyrrole

The relative stability of the isomeric σ-complexes formed in the sulfonation of pyrrole at the α-or β-position (the α-isomer is energetically more favorable) does not agree with the experimentally established positional selectivity of substitution (the formation of β-pyrrolesulfonic acid). However, quantum-chemical calculations of the energy parameters for the reaction of pyrrole and SO₃ with due regard to the solvation effect in the model solvent methylene chloride (ε = 8.93) lead to the conclusion that the calculated activation energy of the rearrangement to the more favorable β-pyrrolesulfonic acid for the less favorable β-isomer of the σ-complex is lower than on the path to the formation of the α-pyrrolesulfonic acid. It was shown that the significant increase in the polarity of the model medium in the transition to DMSO (ε = 46.7) does not lead to substantial change in the energy parameters of the reaction. The explanation for the positional selectivity during the sulfonation of pyrrole using Py·SO₃, according to previous data, involves the participation of the pyridine in the transformation of the σ-complexes into the products. The calculations were made by the B3LYP/6-31G(d) and HF/3-21+G methods using the model of overlapping spheres to take account of solvation. Dedicated to E. J. Lukevics on his 70th birthday __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 34–40, January, 2007.     

Synthesis and solvent dependence of the photophysical properties of [60]fullerene-sugar conjugates

A method for the synthesis of optically pure C₆₀ derivatives containing one or two d-galactose or d-glucose units is described. It involves the synthesis of sugar-malonate derivatives followed by a cyclopropanation reaction with C₆₀. The solvent dependence of the photophysical properties of the methano[60]fullerene-sugar derivatives was studied using nanosecond laser flash photolysis coupled with kinetic UV-vis absorption spectroscopy and time-resolved singlet oxygen luminescence measurements. The triplet properties of these fullerenes, including transient absorption spectra, molar absorption coefficients and quantum yield for the photosensitised production of ¹O₂ were determined in toluene, benzonitrile and acetonitrile solutions. The transient absorption spectral profiles are solvent independent although small differences are observed in the transient absorption maximum: 720±5 nm for toluene, 710±5 nm for benzonitrile and 700±5 nm for acetonitrile. Triplet state molar absorption coefficients (ε_T) of C₆₀ derivatives vary from 9456±2090 M⁻¹ cm⁻¹, for compound 10 in toluene, and 15,272±4462 M⁻¹ cm⁻¹, for compound 6 in acetonitrile. Triplet state lifetimes (τ_T) for methano[60]fullerene-sugar derivatives, under our experimental conditions, are similar in toluene or benzonitrile solutions (47.5±1.1 μs≤τ_T≤51.4±2.0 μs) but are lower in acetonitrile solutions (31.8±0.6 μs≤τ_T≤43.0±1.1 μs). Toluene and benzonitrile solutions of C₆₀ derivatives have Φ_Δ close to unity.     

The effect of deuterium on the photophysical properties of DNA-stabilized silver nanoclusters

We investigated the effect of using D₂O versus H₂O as solvent on the spectroscopic properties of two NIR emissive DNA-stabilized silver nanoclusters (DNA–AgNCs). The two DNA–AgNCs were chosen because they emit in the same energy range as the third overtone of the O–H stretch. Opposite effects on the ns-lived decay were observed for the two DNA–AgNCs. Surprisingly, for one DNA–AgNC, D₂O shortened the ns decay time and enhanced the amount of µs-lived emission. We hypothesize that the observed effects originate from the differences in the hydrogen bonding strength and vibrational frequencies in the two diverse solvents. For the other DNA–AgNC, D₂O lengthened the ns decay time and made the fluorescence quantum yield approach unity at 5 °C.

We investigated the effect of using D₂O versus H₂O as solvent on the spectroscopic properties of two NIR emissive DNA-stabilized silver nanoclusters (DNA–AgNCs).     

Theoretical study of the effects of solvent environment on photophysical properties and electronic structure of paracyclophane chromophores

We use first-principles quantum-chemical approaches to study absorption and emission properties of recently synthesized distyrylbenzene (DSB) derivative chromophores and their dimers (two DSB molecules linked through a [2.2]paracyclophane moiety). Several solvent models are applied to model experimentally observed shifts and radiative lifetimes in Stokes nonpolar organic solvents (toluene) and water. The molecular environment is simulated using the implicit solvation models, as well as explicit water molecules and counterions. Calculations show that neither implicit nor explicit solvent models are sufficient to reproduce experimental observations. The contact pair between the chromophore and counterion, on the other hand, is able to reproduce the experimental data when a partial screening effect of the solvent is taken into account. Based on our simulations we suggest two mechanisms for the excited-state lifetime increase in aqueous solutions. These findings may have a number of implications for organic light-emitting devices, electronic functionalities of soluble polymers and molecular fluorescent labels, and their possible applications as biosensors and charge/energy conduits in nanoassemblies.     

Thorpe-Ingold effect on the conformation and photophysical properties of dialkylsilylene-spaced divinylarene copolymers

Geminal disubstitution on silicon in dialkylsilylene-spaced divinylarene copolymers may dictate the conformation and photophysical properties of the copolymers, bulky (i)Pr substituted copolymers being more folded than Me substituent analogues.     

Effects of heterocyclic ring and amino-ethyl-amino group on the electronic and photophysical properties of a triphenylamine-pyrimidine dye

Introduction of a heterocyclic ring and an amino-ethyl-amino group to donor-acceptor (D-A) type photosensitive dyes can modulate the lifetime of the charge separation generated in the D-A dyes as well as their electronic and UV-vis absorption properties. Here we perform density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations to study 11 derivatives of a triphenylamine-pyrimidine, namely MTPA-Pyc, in order to improve the performance of MTPA-Pyc as solar cell sensitizers. Five heterocyclic rings and an amino-ethyl-amino group were introduced on the styryl moiety of MTPA-Pyc. The results show that the introduction of heterocyclic rings generally causes an absorption red shift, but the absorption intensity reduces as a result of the increase in the dihedral angle between the donor and acceptor. Further, introduction of an amino-ethyl-amino group to these dyes with a heterocyclic ring modification disrupts the conjugation between the donor and acceptor, which does not benefit the absorption but may have the potential to increase the lifetime of charge separation of the dyes. We identify 2 out of 11 dyes that have the best potential for solar cell applications.     

Substituent effect on the photophysical properties, electrochemical properties and electroluminescence performance of orange-emitting iridium complexes

A series of bis(2-phenylbenzothiozolato-N,C(2'))iridium(acetylacetonate) [(bt)(2)Ir(acac)] derivatives, 1-4, were synthesized. Different substituents (CF(3), F, CH(3), OCH(3)) were introduced in the benzothiazole ring to study the substituent effect on the photophysical, electrochemical properties and electroluminescent performance of the complexes, and finally to select high-performance phosphors for use in organic light-emitting diodes (OLEDs). All complexes 1-4 and (bt)(2)Ir(acac) are orange-emitting with tiny spectral difference, despite the variation of the substituent. However, the phosphorescent quantum yield increases with the electron-withdrawing ability of the substituent. This is in contrast to the previous observation that the substituent in the phenyl ring bonded to the metal center of (bt)(2)Ir(acac) not only affected the luminescent quantum efficiency but also greatly tuned the emission color of the complexes. Quantum chemical calculations revealed that the substituents in this position do not make a significant contribution to both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which probably accounts for the fact that they do no strongly influence the bandgap and emission color of the complexes. Orange OLEDs were fabricated using 1-4 as doped emitters. The electron-withdrawing CF(3) and F groups favor improving the electroluminescence efficiency in comparison with that of the parent (bt)(2)Ir(acac), while electron-donating CH(3) and OCH(3) are not favorable for light emission. The complex 1 based OLED exhibited a maximum luminance efficiency of 54.1 cd A(-1) (a power efficiency of 24 lm W(-1) and an external quantum efficiency of 20%), which are among the best results ever reported for vacuum deposited orange OLEDs so far.     

Effects of solvent, pH and beta-cyclodextrin on the photophysical properties of 4-hydroxy-3,5-dimethoxybenzaldehyde: intramolecular charge transfer associated with hydrogen bonding effect

The photophysical properties of 4-hydroxy-3,5-dimethoxybenzaldehyde (HDMB) in various solvents, pH and in aqueous beta-cyclodextrin (CD) have been investigated. In non-polar solvents, HDMB gives only one emission maxima; whereas, in polar solvents it shows a dual luminescence. The increase in Stokes shift with increase in polarity is much more for longer wavelength (LW) than for a shorter wavelength (SW) band. This behaviour indicates the formation of an intramolecular charge transfer (ICT) state through relaxation from the normal excited state. Especially in water, the ICT emission is further red shifted to 430 nm with the normal emission band at 330 nm and the relative fluorescence intensities between 330 nm and 430 nm emission bands are affected by the excitation wavelength. However, this excitation wavelength dependence is not large in aqueous beta-CD solutions. These results suggest that the ICT state in polar solvents/water is stabilized through exciplex formation by the hydrogen-bonding interaction between the carbonyl group and polar solvents/water. The ground and excited state pK(a) values for the neutral-monoanion equilibrium have been measured and discussed. HDMB forms a 1:1 inclusion complex with beta-CD. A mechanism is proposed to explain the inclusion process.     

The effect of solvent on the catalytic properties of microperoxidase-11

The effect of a range of solvents on the catalytic oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide by MP-11 and by a cyclodextrin derivative of MP-11 was examined. The addition of low concentrations of alcohols enhanced the initial rate of sulfoxidation, most likely due to dispersion of MP-11 aggregates. Higher alcohol concentrations resulted in a decrease in activity arising from solvation of the hydrophobic sulfide, disrupting binding to the catalyst. In alcohols, the yield of product was decreased due to increased rates of MP-11 deactivation via the formation of aldehydes (for primary alcohols) or by peroxide-based deactivation. The catalytic activity of the cyclodextrin modified MP-11 was similar to that of MP-11 itself, demonstrating that it is the N-terminal side of MP-11 which is the determinant of catalytic activity.